1. Field of the Invention
This invention relates to improved materials and processes for photowriting optical diffraction gratings into glasses. More particularly, this invention relates to fast processes for photowriting permanent high efficiency optical gratings by exposing glasses previously darkened by high energy radiation to interfering writing laser beams to photobleach these gratings into the glasses.
2. Description of the Related Art
Optical diffraction gratings, both in fiber and bulk form, are finding their way into an ever-growing number of applications, including communications systems, strain sensing systems, and optical processing and computing systems.
Generally speaking, the effectiveness of an optical grating is measured by its peak-to-valley index modulation (.DELTA.n), or by its scattering efficiency (.eta.). The refractive .DELTA.n is related to .eta. by the expression: EQU .DELTA.n=(1/.pi.)(.lambda./l)(.eta.).sup.1/2
where l is the sample thickness and .lambda. is the wavelength of the probe light.
Optical gratings in glasses are most typically laser-induced, i.e. photowritten into glasses by interfering laser beams. See, e.g., Durville et al., "Laser-induced refractive-index gratings in Eu-doped glasses", Physical Rev. B 34 (6) (Sep. 15, 1986), which is incorporated by reference herein.
This method is slow; a typical grating requires anywhere from several minutes to several dozen minutes to write using this method. The gratings produced by this method are not particularly strong; peak-to-valley index modulations (.DELTA.n) of about 5.times.10.sup.-7 are typical, as noted above.
It has been demonstrated that stronger gratings (.DELTA.n on the order of 3.times.10.sup.-5) can be written in germanosilicate glasses. See Meltz et al., "Formation of Bragg gratings in optical fibers by a transverse holographic method", Optics Letters 14 (15) pp. 823-25 (Aug. 1, 1989). However, Meltz et al. achieve this high An by using a large, expensive (several hundred thousand dollars), high-power (with instantaneous beam intensities on the order of 10.sup.7 W/cm.sup.2), high energy (.lambda. on the order of 244 nm) doubled excimer-pumped dye laser. This dye laser system was used as a work-around to the notoriously poor beam quality inherent in excimer lasers.
It is desired to make high An gratings using small, inexpensive, low power, continuous wave visible-light lasers with high beam quality (i.e. high transverse and longitudinal coherence), such as argon lasers. It is also desired to make gratings with high .DELTA.n in glasses other than germanosilicate. In particular, it is desired to make gratings with high .DELTA.n in a broad range of silicate glasses (a silicate glass is defined herein to be any glass where silica is the predominant glass former).